[BluetoothLEHelper] reset notify / indicate state whenever a device gets disconnected

[GeertvanHorrik]

Related to #2871 and #2873

Whenever a device gets disconnected, the calls to SetNotify and SetIndicate will fail because the internal state will still assume they are set, but that's impossible if a device was disconnected.

[GeertvanHorrik]

Important: StopIndicateAsync and StopNotifyAsync should always reset both values (since it specifies None).

https://github.com/windows-toolkit/WindowsCommunityToolkit/blob/master/Microsoft.Toolkit.Uwp.Connectivity/BluetoothLEHelper/ObservableGattCharacteristics.cs#L370

Also note that whenever indicate is used, notify should be reset and vice versa.